Surgical alteration of the shape of a patient's cornea to improve visual acuity has been practiced for a number of years. Such surgery, termed "radial keratotomy", involves the making of radial cuts into the surface of the cornea so as to alter its external curvature. More recently, excimer lasers have been employed to ablate sections of a patient's cornea to modify its optical properties (termed "laser keratectomy"). Such laser-based surgery may be preceded by ultrasonic imaging of both the patient's corneal surface and a measurement of thickness variations of the cornea across its surface. Knowledge of thickness variations of individual corneal layers enables the surgeon to apply proper levels of laser power during the surgical procedure.
Refractive corneal surgery depends on accurate corneal shape alterations performed relative to the visual axis. In using ultrasound analysis (or any other system) to determine corneal micro-anatomy and shape, it is preferable that the ultrasound image be centered relative to the visual axis. In this way, the imaging study and the corneal surgical procedure will be similarly aligned, and pre- or post-operative studies can be reliably correlated. Applicants' system is intended to produce alignment of the visual axis to the vertical, and thus produce exactly the same positioning of the eye (relative to the earth) whether pre-,intra- or post-operative. The prior art has attempted to accomplish alignment of visual axes by asking the patient, while in a supine position, to focus on a light spot projected on the ceiling. Due to the relative closeness of the projected light spot, this procedure results in patient's visual axes always being somewhat convergent.
The prior art includes a number of different systems for both measuring inter-pupillary distance and for localizing the position of the visual axes of the eyes for placement of spectacle lenses. Such apparatus may be found in the following patents: 1,327,163 of Mathewson et al; 2,197,139 of Warner; 2,361,534 of Eppenstein; 2,477,518 of Kappauf et al; 2,596,264 of Macbeth; 4,057,054 of Giannone; 4,653,881 of Joncour and 4,838,676 of Buget et al. Each of the noted patents describes apparatus which must be placed in front of the eye and would obscure the eye (and its internal structures) from an ultrasound scanner.
Accordingly, it is an object of this invention to provide an apparatus and method for assuring parallel and vertical alignment and fixation of a patient's visual axes so as to enable standardization of the position of the visual axis for ultrasonography and laser treatment of the eye while the visual axes are in a known position.
It is another object of this invention to provide an improved method for assuring vertical and parallel alignment of visual axes of a patient's eyes, wherein convergence or divergence is avoided.